Safety system for a lift truck

ABSTRACT

A safety system for a lift truck wherein the hydraulic pressure and thus the load capacity of the truck is automatically changed as a function of the position of the load carriage above the ground level. The safety system include pressure limiting valves which are controlled by the load carriage during its movement up and down the truck mast.

United States Patent 11 1 Bredberg 1 Nov. 27, 1973 [54] SAFETY SYSTEM FOR A LIFT TRUCK 2,810,930 10/1957 MacDonald 91/400 3,125,856 3/1964 Branson 91/451 Inventor! g Bredbelg, MJolby, 3,307,656 3/1967 Susag 187/9 Sweden 3,542,161 11 1970 ulinski 187/9 [73] Assignee: AB Bygg-Och, Transportekonomi (BT), Sweden Primary Examiner-Harvey C. Homsey Filed: June 1971 Att0rneyDav1d Toren et al.

Appl. 190.: 157,983

Foreign Application Priority Data July 1, 1970 Sweden 9101/70 US. Cl .l 187/9, 91/400, 91/451 Int. Cl 1366b 9/20 Field of Search 187/9, 17; 91/400,

References Cited UNITED STATES PATENTS 6/1956 Remde 187/9 ABSTRACT A safety system for a lift truck wherein the hydraulic pressure and thus the load capacity of the truck is automatically changed as a function of the position of the load carriage above the ground level. The safety system include pressure limiting valves which are controlled by the load carriage during its movement up and down the truck mast.

2 Claims, 8 Drawing Figures PATENTED NOV 2 71975 FIGZ SAFETY SYSTEM FOR A LIFT TRUCK The present invention relates to means for making lift truck operation safe and particularly to safety means to prevent truck lift stability from reaching an unsafe condition.

There are certain standard specifications as to the stability which a truck must meet, so that it can be used in a safe and proper way. In practice the load capacity is allowed to be constant up to a certain level and then successively decreases up to the maximum lift level of the load carriage.

It is a main object of the present invention to provide a lift truck with means to automatically change the load capacity as a function of the position of the load carriage above the ground level.

This and other objects and advantages of my invention will more fully appear in connection with the accompanying drawings, wherein FIG. 1 is a sectional view of a three-stage lift truck mast in a retracted positon,

FIG. 2 is a sectional view of the same mast but in a wholly extended position,

FIG. 3 shows a diagram of the hydraulic system truck having a stageless limitation of the load capacity,

FIG. 4 shows a diagram of the load capacity as a function of the position of the load carriage of a truck provided with a stageless limitation of the load capacity according to FIG. 3,

FIG. 5 shows a diagram of the hydraulic system for a lift truck having a stepped limitation of the load capacity,

FIG. 6 shows a diagram of the load capacity as a function of the position of the load carriage of a truck provided with a stepped limitation of the load capacity according to FIG. 5,

FIG. 7 is a view of the mast in the retracted position, and

FIG. 8 is a view of the mast in an extended position.

The truck according to the invention is provided with a hydraulic system having one or more valves, which fix the limits of the hydraulic pressure allowed for the moment. These valves can be operated mechanically or electrically. An example of such a hydraulic system is shown as a principle diagram in FIG. 3. The system consists mainly of a hydraulic pump 1, driven by an electric motor 2, a fixedly set pressure limiting valve 3, a control valve 4, manually set by the operator, an oneway acting hydraulic lift cylinder 5 and a hydraulic oil tank 6. Further is in the pressure conduit parallel to the valve 3 inserted an adjustable pressure limiting valve 7, provided with a return conduit 16 to the tank This mast is connected to the truck at 14. The lift mast,

which in the present example of the invention can be extended from a wholly retracted position according to FIG. 1 to an extended position according to FIG. 2, is provided with a movable load carriage, in this case in the form of a pair of forks 9. These can be lifted from their lowest position according to FIG. 1 and up to the highest position according to FIG. 2. The movement of the forks is in a known way effected by the hydraulic cylinder 5 and the chains 13.

The pressure hydraulic cylinder 5 is directly proportional to the weight of the load resting on the forks 9. As mentioned above the load capacity is not always constant for the whole lift height of the truck but must be reduced at lifts above a certain level. Thus the machine cannot be allowed to lift full load up to its maximum lift height. The variation of the load capacity with the lift height may look like what is shown in FIG. 4. Up to a certain level the load capacity is constant and after that decreases vrectilinearly up to the maximum lift height of the machine, where the load capacity has reached its minimum. The allowed hydraulic pressure must then be adapted to the position of the load carriage above the ground. According to the invention this adaptation is made automatically and either rectilinear or step-by-step. A rectilinear embodiment is the one which comprises the adjustable pressure limiting valve 7 according to FIG. 3. The indication of the height of the load carriage above the ground is performed by means of a wire 10, which is secured to the load carriage 9 at 11 and runs via a number of guide rollers 12 to the adjustable pressure limiting valve 7.

Up to the height, where the limitation of the load capacity shall start, the setting of the valve 3 determines the load capacity, this valve for the range in question being set for a lower pressure than the adjustable valve 7. For the range thereabove the latter valve takes over the function as load limiter, as it by means of the wire 10 is adjusted to a lower pressure than the fixedly set valve 3 and to a successively lower and lower pressure the higher up the load carriage 9 has been moved. The successive adjustment of the valve 7 then follows the right line shown in FIG. 4.

The curve according to lastrnentioned figure can be varied entirely as desired. The maximum pressure can be set by means of the fixed valve 3. The point where the adjustable valve 7 shall take over' the capacity limiting function and also the inclination of the curve can be varied by means of the setting and characteristic of the adjustable valve '7.

The indication of the position of the load carrier above the ground can be made in several ways. It is not necessary to have the sensing means on the load carriage but it can be arranged on any other mast part,,the movement of which corresponds to that of the load carriage. This control member can also be in the form of an endless chain, which runs over an upper and lower guide roller and is secured to any mast part moving in correspondence to the movement of the load carriage. The movement of the chain is by means of a toothed wheel transferred to the pressure limiting valve 7.

An embodiment which step-by-step adapts the maximum hydraulic pressure of the system to the level of the load carriage above the ground can be made in accordance with the FIGS. 5 and 6. Here is a number of fixedly set pressure limiting valves, which are arranged in parallel and have feed conduits supplied with electrically controlled stop valves 17, Hand 19. These are normally shut-of, but are opened when the load carriage reaches the lift heights h h and h of which h is the lowest one. The opening is effected by limit switches 17a, 18a and 19a placed on the mast 8, these switches closing a circuit which furnishes the magnet coils of the valves with current from the battery of the truck. The pressure limiting valves 20, 21 and 22 are set on various pressures p p and p of which p is the highest one. When the load carriage 9 reaches the lift height h the location of limit switch the valve 17 opens the feed conduit to the pressure limiting valve 20, which then reduces the maximum allowed pressure from p to p When the load carriage 9 reaches the lift height h the location of limit switch 18a, the valve 18 opens the feed conduit to the pressure limiting valve 21, which then reduces the maximum allowed pressure further to p and when the load carriage 9 reaches the lift height k the location of limit switch 19a, the valve 19 opens the feed conduit to the pressure limiting valve 22, which then reduces the pressure further to p which is maintained up to the maximum lift height h and which corresponds to the maximum load which can be lifted up to maximum lift height. When using this method part of the capacity is lost, viz. the range between the stepped line and the dotted-dash line, which is the line corresponding to the maximum allowed pressure for each actual lift height. The more pressure limiting valve the smaller are the steps and the stepped line will then come closer to the optimum pressure line.

Having described my invention, 1 claim:

1. A hydraulic control system for a lift truck including a mast and load carriage mounted for vertical movement upon said mast, said system comprising:

a hydraulic lift cylinder operatively connected to drive said load carriage in response to fluid pressure applied thereto;

a source of hydraulic pressure fluid;

hydraulic pump means for applying fluid pressure from said source to said lift cylinder;

pressure limiting valve means connected between said pump means and said lift cylinder for limiting to a predetermined maximum the pressure applied to said lift cylinder;

adjustable pressure control valve means connected between said lift cylinder and said pressure fluid source for controlling the pressure applied to said lift cylinder in dependence upon the vertical position of said load carriage and for diminishing said pressure in a predetermined relationship with the vertical position of said load carriage;

means for sensing the vertical location of said load carriage; and

means for enabling control of the operation of said pressure limiting valve means and of said adjustable pressure control valve means in response to said sensing means whereby pressure applied to said load cylinder is maintained by said pressure limiting valve means at a predetermined maximum up to a given vertical position of said load cylinder and whereby said pressure is diminished in a predetermined relationship by said adjustable pressure control valve means for vertical positions of said lift cylinder above said given verticle position.

2. A hydraulic control system according to claim 5, wherein said adjustable pressure control valve means comprise a plurality of valves connected in parallel with each other and with said pressure limiting valve means, said plurality of valves each being responsive to said sensing means for operating in a successive manner to diminish the pressure applied to said lift cylinder in a stepped predetermined relationship as said load carriage moves above said given vertical level. 

1. A hydraulic control system for a lift truck including a mast and load carriage mounted for vertical movement upon said mast, said system comprising: a hydraulic lift cylinder operatively connected to drive said load carriage in response to fluid pressure applied thereto; a source of hydraulic pressure fluid; hydraulic pump means for applying fluid pressure from said source to said lift cylinder; pressure limiting valve means connected between said pump means and said lift cylinder for limiting to a predetermined maximum the pressure applied to said lift cylinder; adjustable pressure control valve means connected between said lift cylinder and said pressure fluid source for controlling the pressure applied to said lift cylinder in dependence upon the vertical position of said load carriage and for diminishing said pressure in a predetermined relationship with the vertical position of said load carriage; means for sensing the vertical location of said load carriage; and means for enabling control of the operation of said pressure limiting valve means and of said adjustable pressure control valve means in response to said sensing means whereby pressure applied to said load cylinder is maintained by said pressure limiting valve means at a predetermined maximum up to a given vertical position of said load cylinder and whereby said pressure is diminished in a predetermined relationship by said adjustable pressure control valve means for vertical positions of said lift cylinder above said given verticle position.
 2. A hydraulic control system according to claim 5, wherein said adjustable pressure control valve means comprise a plurality of valves connected in parallel with each other and with said pressure limiting valve means, said plurality of valves each being responsive to said sensing means for operating in a successive manner to diminish the pressure applied to said lift cylinder in a stepped predetermined relationship as said load carriage moves above said given vertical level. 